Dense phase gas photochemical process for substrate treatment

ABSTRACT

A process for removing undesired material from a chosen substrate by exposing the substrate simultaneously to ultraviolet radiation and a selected dense fluid, wherein the radiation produces a photochemical reaction that removes the undesired material from the substrate and the dense fluid enhances the removal of the undesired material. In an alternative embodiment, a reactive agent may additionally be used. The process may be used to remove contaminants from a substrate, etch a substrate surface, or destroy toxic organic material in industrial wastes.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a process for treating asubstrate in order to remove undesired material therefrom by exposingthe substrate simultaneously to radiation and a dense fluid. Moreparticularly, the present invention relates to methods for removingcontaminants from a substrate, for etching a substrate surface, and fordetoxifying or decontaminating industrial waste materials.

2. Description of Related Art

In the fabrication of various electronic and optical devices, thesurface of the devices often become contaminated with undesiredmaterials, such as organic or inorganic materials, which must besubsequently removed. In addition, in the fabrication of such devices,it is sometimes desirable to etch the surface of the device prior tosubjecting it to further processing, such as oxide layer deposition orpatterned metal layer formation.

As is known in the art of decontamination, ultraviolet radiation,particularly radiation at 253.7 nanometers (nm) and 184.9 nm, isabsorbed by contaminant hydrocarbons and causes dissociation thereof.The fragments produced by this dissociation absorb additionalultraviolet radiation and are further dissociated. Complete dissociationresults in the formation of water, carbon dioxide, or nitrogen.Ultraviolet radiation has been used to decompose organic contaminants inwater and waste water, as disclosed, for example, in the publicationentitled "Investigation into Chemistry of the UV-Ozone PurificationProcess" by E. Leitis, Jan. 31, 1979, Report No. NSF/RA 790038, NationalTechnical Information Service Accession No. PB 296485. However, organicwastes, either solid or liquid, are not usually miscible with water andcannot be treated by standard UV treatment methods. In such cases, theorganic wastes are usually incinerated or buried in a landfill. Inaddition, this type of photolysis process has been used in combinationwith oxidation processes to remove organic contaminants from a varietyof substrates. Ultraviolet/ozone cleaning has been used to removeorganic contaminants from surfaces such as glass, quartz, mica,ceramics, and metals, as described, for example, by L. Zafonte and R.Chiu, in the publication entitled "UV/Ozone Cleaning For OrganicsRemoval on Silicon Wafers," Paper No. 470-19, Proceedings of the SPIE1984 Microlithography Conferences, Mar. 11-16, 1984, Santa Clara, Calif.In such a process, two wavelenqths of radiation are used: 253.7nanometers and 184.9 nm. The 253.7 nm radiation is absorbed by thecontaminant hydrocarbons and produces bond scission. The resultingfragments are further dissociated by additional exposure to the 253.7 nmradiation, and the final products of this dissociation are water, carbondioxide, or nitrogen. Concurrently, the 184.9 nm radiation is absorbedby molecular oxygen present in the air environment and dissociates thelatter to produce atomic oxygen. The atomic oxyqen then combines withadditional molecular oxygen present in the environment to form ozone(O₃). The ozone oxidizes the hydrocarbon contaminants to produce carbondioxide and water as the final products. The ozone also absorbs 253.7 nmradiation and is dissociated into molecular oxygen and atomic oxygen.The latter two species recombine to form ozone. Thus, ozone iscontinually formed and dissociated.

These conventional ultraviolet/ozone processes for cleaning substratesurfaces have several disadvantages. In order for the ultraviolet/ozoneprocess to be effective, it is essential that the substrate be properlyprecleaned to remove gross contaminants. The precleaning steps generallyuse polar and nonpolar solvents, followed by an ultrapure water rinse.The polar solvents remove ionic or inorganic contamination which may notbe removed by the ultraviolet/ozone step; and the nonpolar solventsremove all gross organic contaminants. In addition, since theconventional ultraviolet/ozone cleaning process is designed to removemonolayer surface contaminants, the process is presently limited todecontamination of minute levels of hydrocarbons. Any gross inorganic orionic contaminants which were not removed during the precleaningoperations will not be removed by the photosensitized oxidation whichoccurs during ultraviolet/ozone exposure. Further, all gross hydrocarboncontamination must be removed in a precleaning step since the exposureof gross hydrocarbon contamination to ultraviolet radiation will resultin cross linking and charring. Such polymerization and charring productscreate a shield for buried layers of contaminants and terminate thecleaning process.

Other problems with the conventional ultraviolet/ ozone cleaning arethat the process is performed in a gaseous environment at atmosphericpressure or reduced pressure, is based solely on photodegradation, andlimited ozone contact, and does not include solvation and fluid transfermechanisms. The previously described oxidation process occurs in agaseous environment, which is non-intimate and difficult to stabilize.The previously described dissociation of contaminants takes place onlyon the surface. In addition, only contaminant by-products which aregaseous are removed. Finally, the conventional ultraviolet/ozonecleaning process is limited in that the surfaces to be cleaned must bein line of sight to the ultraviolet light source for effectivephotochemical degradation. Thus, surfaces with cavities and holes orsurfaces located beneath surface mounted components cannot beeffectively cleaned.

Another type of cleaning system involves the use of dense phase gases asa replacement for conventional organic solvents. A dense phase gas is agas compressed under either supercritical or subcritical conditions toliquid-like densities. These dense gases are referred to as densefluids. Unlike organic solvents, such as n-hexane, or1,1,1-trichloroethane, dense phase gas solvents exhibit unique physicaland chemical properties such as low surface tension, low viscosity, andvariable solute carrying capacity.

The solvent properties of compressed gases is well known. In the late1800's, Hannay and Hogarth found that inorganic salts could be dissolvedin supercritical ethanol and ether (J. B. Hannay and H. Hogarth,J.Proc.Roy.Soc. (London), 29, p. 324, 1897). By the early 1900's,Buchner discovered that the solubility of organics such as naphthaleneand phenols in supercritical carbon dioxide increased with pressure (E.A. Buchner, Z.Physik.Chem., 54, p. 665, 1906). Within forty yearsFrancis had established a large solubility database for liquified carbondioxide which showed that many organic compounds were completelymiscible (A. W. Francis, J.Phys.Chem., 58, p. 1099, 1954).

In the 1960's there was much research and use of dense gases in the areaof chromatography. Supercritical fluids (SCFs) were used as the mobilephase in separating non-volatile chemicals (S. R. Springston and M.Novotny, "Kinetic Optimization of Capillary Supercritical Chromatographyusing Carbon Dioxide as the Mobile Phase", CHROMATOGRAPHIA, Vol. 14, No.12, p. 679, Dec. 1981).

Documented industrial applications utilizing dense fluid cleaninginclude extraction of oil from soybeans (J. P. Friedrich and G. R. Listand A. J. Heakin, "Petroleum-Free Extracts of Oil from Soybeans", JAOCS,Vol. 59, No. 7, July 1982), decaffination of coffee (C. Grimmett,Chem.Ind., Vol. 6, p. 228, 1981), extraction of pyridines from coal (T.G. Squires, et al, "Supercritical Solvents. Carbon Dioxide Extraction ofRetained Pyridine from Pyridine Extracts of Coal", FUEL, Vol. 61, Nov.1982), extraction of flavorants from hops (R. Vollbrecht, "Extraction ofHops with Supercritical Carbon Dioxide", Chemistry and Industry, 19 June1982), and regeneration of absorbents (activated carbon) (M. Modell,"Process for Regenerating Absorbents with Supercritical Fluids", U.S.Pat. No. 4,124,528, 7 Nov. 1978).

As the complexity of manufactured devices and structures increases andcleanliness requirements for such devices and structures increase, moreeffective and more efficient cleaning methods are required.Electro-optical devices, lasers and spacecraft assemblies are fabricatedfrom many different types of materials having various internal andexternal geometrical structures which are generally contaminated withmore than one type of contamination. (These highly complex and delicateassemblies can be classified together as "complex hardware".)Consequently, there is a continuing need to provide improved cleaningprocesses in which both gross and precision cleaning are simultaneouslyaccomplished.

With regard to surface preparation, such as in the fabrication ofelectronic devices, known methods include both chemical and physicalmeans for removing the surface layer from the substrate prior todeposition. Such methods include, for example, wet chemical etching withaqueous or non aqueous materials, plasma etching, or ultrasonics. Eachof these methods has the disadvantages that it requires expensiveequipment, uses solvents, and must be performed as a separate processingstep. Thus, there is a continuing need to provide improved methods forsubstrate surface preparation prior to deposition of a material on thesubstrate.

With regard to the treatment of industrial waste materials, knownmethods for treating solid hazardous organic wastes include thermaldecomposition, pyrolysis, or UV-peroxidation. These methods have thedisadvantage that they require burning, reaction, or stabilization ofthe solid waste prior to disposal. Thus, there is a continuing need toprovide improved methods for the treatment of industrial waste materialsto remove or destroy unwanted solid organic or inorganic materials,particularly toxic materials.

SUMMARY OF THE INVENTION

The general purpose of the present invention is to provide a new andimproved method for removing undesired material from a chosen substrate.This process possesses all of the advantages of the above prior artprocesses while overcoming most of their significant disadvantages.

The above general purpose of this invention is accomplished by firstplacing the substrate containing the undesired material in a cleaningvessel. The contaminated substrate is contacted with a chosen densefluid and simultaneously with radiation of a predetermined wavelength.The radiation induces a photochemical reaction which removes theundesired material from the substrate and the dense fluid enhances thisremoval of the undesired material.

In accordance with one embodiment of the present invention, acontaminant is removed from the surface of a substrate. In accordancewith a second embodiment of this invention, a portion of the substratesurface is removed in an etching process. In accordance with a thirdembodiment of this invention, organic contaminants are removed fromindustrial waste materials. In accordance with an alternative embodimentof the present invention, a dense phase oxidant is used to enhance theremoval of the undesired material.

These and many other features and attendant advantages of the presentinvention will become apparent as the invention becomes betterunderstood by reference to the following detailed description whenconsidered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a system for practising apreferred exemplary process of the present invention in which the densefluid comprises a compressed gas.

FIG. 2 is a schematic representation of a system for practising anexemplary alternative embodiment of the present invention in which thesubstrate comprises slurried solid industrial waste containing organiccontaminants.

FIG. 3 presents a graph comparing the percent of surface organiccontaminants removed by the process of the present invention, by aconventional ultraviolet/ozone process, and by a conventional densefluid cleaning process.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with the process of the present invention, a substratecontaining an undesired material, such as organic contaminants, isexposed simultaneously to a selected dense fluid and radiation of aselected wavelength. The dense fluids suitable for use in the presentprocess comprise either supercritical or liquified gases, or inorganicliquids at standard temperature and pressure (STP). Suitable gasesinclude any of the known gases which may be converted to supercriticalfluids or liquified at temperatures and pressures which will not degradethe physical or chemical properties of the substrate being cleaned, ifthe latter is of concern, such as for delicate aerospace hardware. Onthe other hand, if the substrate comprises waste material, extremetemperatures and pressures may be used, if required, to form thesupercritical fluids. These gases typically include, but are not limitedto: (1) hydrocarbons, such as methane, ethane, propane, butane, pentane,hexane, ethylene, and propylene; (2) halogenated hydrocarbons such astetrafluoromethane, chlorodifluoromethane, sulfur hexafluoride, andperfluoropropane; (3) inorganics such as carbon dioxide, ammonia,helium, krypton, argon, xenon, and nitrous oxide; and (4) mixturesthereof. The term "dense phase gas" as used herein is intended toinclude mixtures of such dense phase gases. The dense phase gas selectedto remove a particular contaminant is chosen to have a solubilitychemistry which is similar to that of the targeted contaminant. Forexample, if hydrogen bonding makes a significant contribution to theinternal cohesive energy content, or stability, of a contaminant, thechosen dense phase gas must possess at least moderate hydrogen bondingability in order for solvation to occur. In some cases, a mixture of twoor more dense phase gases may be formulated in order to have the desiredsolvent properties, as discussed hereinbelow with regard to analternative embodiment of this invention. The selected dense phase gasmust also be compatible with the substrate being cleaned, and preferablyhas a low cost and high health and safety ratings. Further, it ispreferred that the selected dense phase gas not dissociate when exposedto the selected radiation used in the present process, or if it doesdissociate, it forms products which are useful and desired in thepresent process, as described hereinbelow. In particular, organic gasessuch as freon and hexane are not desirable as the major dense phase gasfor the present invention although they may be used in small quantitiesto produce desired radicals as described hereinbelow.

Carbon dioxide is a preferred dense phase gas for use in practising thepresent invention since it is inexpensive and non toxic. The criticaltemperature of carbon dioxide is 305° Kelvin (32° C.) and the criticalpressure is 72.9 atmospheres (75 kilograms per square centimeter,Kg/cm²). At pressures above the critical point, the phase of the carbondioxide can be shifted between the liquid phase and supercritical fluidphase by varying the temperature above or below the critical temperatureof 305 Kelvin (K).

Liquids which are suitable for practising the present process include,for example, ultrapure water (i.e., having a resistance of greater than10 megohms cm), and optionally may have a selected gas, such as oxygenor carbon dioxide dissolved therein. Ultrapure water having dissolveddense gas therein is also suitable for use as the dense fluid of thepresent invention. The pressure of the gas in the water is preferablybetween about 72.9 and 250 atmospheres (75 and 258 Kg/cm²) As anexample, carbon dioxide compressed to 75 atmospheres (77 Kg/cm²) at 35°C. (308° K.) over the water produces a two-phase cleaning system. Thephases may be liquid carbon dioxide/water or supercritical carbondioxide/ water, depending on whether the temperature is, respectively,below or above the critical temperature of carbon dioxide.

The radiation used in practising the present process is selected toproduce the dissociation of the undesired material or contaminant. Thepreferred radiation comprises ultraviolet (UV) radiation within therange of 184 to 300 nm. Such radiation is produced by commerciallyavailable mercury or xenon lamps. Preferred wavelengths within thisrange are about 184.9 and 253.7 nm. High energy pulsed radiation can beused if desired provided that it is suitable for cleaving contaminantbonds. In some cases, the radiation may alter the molecular structureand properties of the dense fluid so as to enhance its cleaning ability.For example, photoexcited dense phase ozone is expected to be highlyenergetic. In addition, spectroscopic evidence indicates that densephase carbon dioxide is non linear and hence polar duringphotoexcitation.

In accordance with the present process, the radiation produces bondscission or dissociation of the contaminant on the substrate aspreviously described. However, the simultaneous use of a dense fluidwith this radiation produces an unexpected improvement in theeffectiveness of the cleaning process. The dense fluid is believed toenhance the radiation propagation to thereby increase surface cleaningof the substrate. Preferably, the refractive index of the dense fluid ormedia should be similar or equal to that of the substrate to therebyeliminate reflection of radiation at the substrate/dense fluidinterface. This matching of refractive indexes increases radiationintensity at the surface of the substrate and enhances contaminantremoval. Further, it is believed that the internal reflection providedby dense fluids causes the radiation to be more effectively transmittedinto holes, cracks, crevices and other surfaces not in direct line-ofsight with the radiation source. Accordingly, the radiation is scatteredthroughout the cleaning vessel exposing all substrate surfaces. Thisphenomenon decreases dependence of substrate distance from the radiationsource for effective surface cleaning.

In addition, the dense fluid used in the present invention is notdegraded by exposure to UV light and thus serves as an effective UVtransmission medium and waste carrier medium. The dense fluid bathes thesubstrate surface and dissolves or suspends the contaminants forsubsequent photolytic dissociation or physical or chemical separation.Further, the dense fluid suspends and transports the products ofphotolysis and thus prevents surface char formation which would reduceexposure of the substrate surface to additional radiation. Finally,dense phase gases are excellent solvents, and their low viscosity andvariable solvent power make them ideal transport media. Contaminants inporous media can be removed from the pores and then once outside thepores can be removed physically. Thus, the present processsimultaneously provides both the precleaning step and the precisioncleaning step which have been performed in separate steps in prior artprocesses as previously discussed. Further, dense fluids provide bettercontrol of cleaning environment parameters than conventional liquidcleaning agents. In aqueous fluids used in the conventional methods,temperature, pH, and conductivity are controlled; whereas in thesupercritical fluids used in the present process, temperature andpressure are controlled. By controlling these latter parameters, thesolvent power of the fluid can be matched with the contaminant tothereby increase the cleaning power. Moreover, the present processrequires shorter treatment time in order to clean the substrate ascompared to conventional cleaning processes. During the photolysis ofthe present invention, larger organic molecules are cleaved into smallerfragments which are easier to solvate or suspend than larger molecules.This increased contaminant solvation or suspension in dense fluidsdecreases surface cleaning time. In addition, in the present process,photodegradation of contaminants takes place in fluid phase rather thanon a solid surface.

In an alternative embodiment of the present invention, a dense phase gasoxidant is additionally used in order to enhance the removal of theundesired material. As is known in the art, an oxidizing gas enhancesthe photodegradation of contaminants. However, in accordance with thepresent process, dense phase oxidants are used. Unlike conventionaloxidant enhancement agents, such as ozone, dense phase oxidants exhibitboth liquid like and gas-like properties. For example, at a temperatureof 25° C. and a pressure of 100 atmospheres (103 Kg/cm²), ozone existsas a supercritical fluid. Supercritical ozone (that is, ozone in asupercritcal state) dissolved in liquid or supercritical carbon dioxideor water, is an excellent solvent/oxidant for inorganic material. Thelow surface tension, low viscosity, and high oxidation potential ofsupercritical ozone dissolved as noted make it an excellent penetratingoxidant and etchant. Other conventional oxidizing gases which have beenused in ultraviolet or photochemical cleaning processes may be used inthe present process. Ozone is the preferred oxidizing fluid for use inthe present process. Oxygen may be provided as a precursor which isconverted to ozone upon exposure to UV radiation. The ozone may beproduced in the cleaning chamber or introduced into the cleaning chamberfrom an external source. In the former case, by precise control of theamount of oxyqen dissolved in the dense fluid, the amount of ozoneproduced may be directly controlled using in situ UV photolysis. Inaddition, other dense fluids which react with the contaminants to aidtheir removal may be used in the present process. One particularlyuseful material is hydrogen peroxide, which can be photodissociated tohydroxyl radicals and peroxide radicals, which react with contaminantsand form innocuous carbon dioxide and water by-products. The hydroxylradicals and/or peroxide radicals may be formed in situ or introducedfrom an external source. Adequate safety precautions must, of course, betaken when using peroxide materials. Further, fluorine gas, which can bephotodissociated to form fluorine, or other dissociablehalogen-containing compounds may be used. Hydrogen gas or ammonia, whichcan be photodissociated to form hydrogen species, may also be used. Thephotodissociated species have increased reactivity with the contaminantand enhance its removal or destruction. Using these reactive densefluids, contaminants in porous media can be removed either chemically oroxidatively. Once outside the pores, the contaminants can be removedeither chemically by the reactive dense fluid or by physical transportby the dense fluid.

The oxidizing or chemically reactive dense fluid may be introduced intothe reaction chamber in a mixture with the main dense fluid used in thepresent process. Optionally, the reactive dense fluid may be introducedinto the reaction chamber by a carrier, such as argon or xenon, which isstable in the presence of radiation.

Contaminant materials which may be removed from substrates in accordancewith the present invention include, but are not limited to, oil, grease,lubricants, solder flux residues, photoresist, particulates comprisinginorganic or organic materials, adhesive residues, plasticizers,unreacted monomers, dyes, or dielectric fluids. Inorganic and organiccontaminants can be removed simultaneously in accordance with thepresent process. Organic contaminants absorb ultraviolet radiationcausing bond cleavage. Inorganic materials are removed through solvationor fluidation in the dense media.

Typical substrates from which contaminants may be removed by the presentprocess include, but are not limited to, substrates formed of metal,carbon, rubber, plastic, cotton, cellulose, ceramics, and other organicor inorganic compounds. The substrates may have simple or complexconfigurations and may include interstitial spaces which are difficultto clean by other known methods. In addition, the substrate may be inthe form of particulate matter or other finely divided material, such ascharcoal. Finally, the substrate may comprise water or other liquidcarrier for waste materials.

In accordance with a first embodiment of the present invention, theundesired material which is removed comprises a contaminant, such as ahydrocarbon material, on the surface of a substrate, such as anintegrated circuit, or a combination of ionic, inorganic and organiccontaminants on the surface of a printed wiring board. In accordancewith a second embodiment of the present invention, the undesiredmaterial which is removed comprises the surface layer of the substrate.Thus, the substrate surface is etched prior to further surface treatmentsuch as metal deposition or bonding. This etching process is uniformsince the contaminant abrasion produced by fluid shear forcescontinually removes ultraviolet char products, exposing buriedcontaminants. In accordance with a third embodiment of the presentinvention, the undesired material which is removed comprises suspendedor dissolved organic contaminants and the substrate (i.e., thesuspension medium) comprises a liquid, such as water, waste water, or adense fluid. Thus, water which has been polluted with organic materialsmay be decontaminated by the process of the present invention. Theultraviolet radiation used in the present process dissociates thecontaminants and thus removes them by destroying them. In this manner,toxic contaminants can be destroyed and the waste material can bedetoxified, which significantly simplifies the disposal of suchmaterial.

FIG. 1 is a schematic representation of a system for practicing apreferred exemplary process of the present invention in which the densefluid comprises a compressed gas. The system includes a vessel 10 inwhich a substrate 12 is positioned for cleaning on a holding device,such as a shelf (not shown). A mercury vapor lamp or xenon flash lamp 14is positioned outside the cleaning vessel 10. A focusing barrel 16 andhigh pressure quartz window 18 are provided to allow introduction of UVradiation into the cleaning zone 20 defined by the vessel 10. A powersupply 22 is provided for energizing the mercury lamp 14. The densefluid in the form of a pressurized dense gas is pumped into the cleaningzone 20 through conduit 24. The dense gas is formed by known methods ofcontrolling temperature and pressure. For this purpose, the cleaningvessel 10 is provided with a heatng/ cooling jacket (not shown) andconduit 24 is provided with a pressure control valve (not shown). Inthis embodiment, line 26 is omitted. The dense fluid containingdissolved and entrained contaminant by products is removed from thecleaning zone 20 through exhaust line 28.

In accordance with an alternative embodiment of the present invention,as previously discussed, a dense phase oxidant or other reactive agentis used to enhance the removal of the undesired material. The system ofFIG. 1 may be adapted for use in this alternative embodiment byincorporating inlet line 26 through which the dense phase oxidant, withor without a carrier gas, is introduced under pressure into the cleaningzone 20. Alternatively, the dense phase oxidant may be mixed with themajor dense phase gas used in the present process and the mixture isthen introduced under pressure through conduit 24 into the cleaningvessel 10. In the latter case, inlet line 26 is omitted. The dense phaseoxidant may comprise, for example, a mixture of carbon dioxide and ozoneor a mixture of carbon dioxide and a predetermined amount of oxygen. Inthe latter case, when the mixture is exposed to radiation at 184nanometers in the cleaning vessel, under dense fluid conditions, densephase ozone is produced.

As previously discussed, the use of ozone in combination withultraviolet radiation improves contaminant photodegradation and materialsurface properties. Accordingly, the amount of oxygen or ozoneintroduced into cleaning zone 20 should be precisely controlled.Preferably, oxygen mixed with carbon dioxide, xenon, or krypton iscompressed to the dense fluid state in the cleaning zone 20 in which theUV radiation converts the dense phase oxyqen to dense phase ozone.Alternately, ozone rather than oxygen may be mixed with the above-notedcarrier gas. Dense phase ozone in the dense phase gas is morehomogeneous and increases solution contact with crevices and holes inthe substrate more effectively than conventional air contact techniques.

A typical cleaning process using the system set forth in FIG. 1 involvesplacing the substrate in the cleaning chamber and then filling thechamber with the desired dense fluid. If an oxidant or other reactiveagent is used, it is introduced into the cleaning zone or produced insitu as previously described. The substrate is then subjected toultraviolet radiation which is transmitted through the dense fluid tothe substrate. After an appropriate exposure time, the ultravioletradiation is turned off, and cleaning ultilizing the dense fluid iseither continued or terminated. The substrate is then removed from thecleaning zone, dried if necessary and packaged. The contaminated densefluid may be regenerated by known means and recycled for use in theabove-described system. For example, the partially decomposedcontaminants may be separated by treating the dense fluid with activatedcarbon, or the pressure of the dense fluid may be reduced below itscritical pressure and the resulting gas may be passed over activatedcarbon or a molecular sieve. Optionally, in a continuous process, thedense fluid containing the contaminants may be removed from the cleaningvessel and replaced with an equal amount of clean (i.e., unexposed)dense fluid.

The process of the present invention may also be used to regeneratespent activated carbon. The system of FIG. 1 is used as described above,using dense phase carbon dioxide and an ozone oxidant. The spent carbonis the contaminated substrate 12. In this process, contaminants aredesorbed from the carbon surfaces through physical/chemical separationand are simultaneously destroyed using dense photochemicaloxidants/fluids (e.g. supercritical ozone). In addition, surfaceadsorbed species are destroyed before they are desorbed. Thus, hazardouswaste by products are destroyed. Some of the carbon is destroyed in thisprocess, but it is only minimal (less than 1%). Typical contaminantswhich may be removed from carbon by the present process include soaps,gasoline, pesticides, hydrocarbons, hydrocarbon gases, andpolychlorinated biphenyls.

FIG. 2 is a schematic representation of a system for practising analternative embodiment of the present invention in which the substratecomprises industrial waste material containing organic contaminants. Thewaste material may be in the form of a slurry containing solidcontaminants or a liquid containing dissolved contaminants. The systemincludes a vessel 30, such as a high pressure stainless steel vessel,which defines a cleaning zone 32. The vessel 30 is provided with aheating/cooling jacket (not shown) in order to control the temperaturein the vessel 30 to provide the required conditions for the dense phasegas. The waste material is introduced into vessel 30 through inlet pipe34. Ultraviolet radiation means 36 produces ultraviolet radiation whichis introduced into the reaction zone 32 by means of a quartz light pipearray 38 which extends into the waste material 40 within the reactionzone 32. A two axis impeller/mixer 42 provides for effective circulationof the waste material being treated. If the waste material comprises aslurry, the mixer also maintains the particulate matter in suspension sothat contact with the dense fluid and ultraviolet light can beoptimized. This mixing may be augmented with an ultrasonic pulse, forexample, at 400 watts/cm, such as produced by a titanium transducerobtained from B. Braun, Biotech, Incorporated in Bethlehem, Pa. If anoxidizing or other reactive agent is used, it is introduced into thereaction zone 32 through inlet pipe 44 or produced in situ as previouslydescribed. Inlet pipe 44 is provided with a pressure control value (notshown) in order to control the pressure in the vessel 30 to provide therequired conditions for the dense phase gas. Treated waste material isremoved through outlet pipe 48. Sampling port 50 is provided in order totake samples of the waste material as it is being treated to determinethe extent of decontamination achieved. The system of FIG. 2 is used inessentially the same manner as the system of FIG. 1.

The system of FIG. 2 may also be used to clean a contaminated substrate,such as an integrated circuit. The substrate is positioned in thecleaning zone 20 on a shelf and is surrounded by the dense fluid, suchas carbon dioxide and water, optionally with a chosen reactive agent.

A study was performed in which the process of the present invention wascompared to the conventional ultra-violet/ozone cleaning andconventional dense fluid cleaning previously described. Four polyimideprinted wiring board samples were exposed to each of the variousprocesses. The first polyimide printed wiring board was used as acontrol specimen. No processing or cleaning was carried out. The samplewas not contaminated with acid flux or exposed to ultraviolet radiation.A second printed wiring board was contaminated with an acid flux. Theboard received no ultraviolet radiation exposure but was soaked fortwenty minutes in deionized water at a flow rate of 150 milliliters perminute (i.e., dense liquid treatment only). A third printed wiring boardwas contaminated with an acid flux and cleaned using a conventionalultraviolet/ozone cleaning system obtained from UVP Inc. of San Gabriel,Calif. The printed wiring board was placed 0.5 inches from theultraviolet source for twenty minutes.

The fourth printed wiring board was contaminated with an acid flux andcleaned using a system as shown in FIG. 1. The dense fluid was ultrapurewater. The temperature of the dense fluid in the cleaning zone wasmaintained at about 40° C. The printed wiring board was held one inchfrom the ultraviolet lamp.

All four of the printed wiring boards were viewed at 100 powermagnification. A visual comparison of the four printed wiring boardsshowed that the board treated in accordance with the present inventionwas cleaned to the nearly original state of the control specimen whereasneither the UV radiation cleaning nor conventional solvent cleaning waseffective in removing the contamination. A graphic representation of theresults of this comparison study is shown in FIG. 3. The graphdemonstrates the synergistic effect of combining ultraviolet radiationcleaning with dense fluid cleaning in accordance with the presentinvention (Line C). Such a combination provides effective cleaning offlux contaminated printed wiring boards while dense fluid cleaning (LineA) or ultraviolet radiation cleaning (Line B) alone was not effective.The use of other dense fluids in place of water in the above describedprocess will provide even greater cleaning effectiveness.

Thus, it can be seen that the present invention provides an improvedmethod for removing contaminants from substrates to increased levels ofcleanliness. This process simultaneously accomplishes precleaning andprecision cleaning steps and removes both organic and inorganiccontaminants. The present process also has the practical advantages ofreducing processing time and costs, and reducing environmental impact bythe regeneration of the operating dense fluids and the lack of organicsolvents.

Having thus described exemplary embodiments of the present invention, itshould be noted by those skilled in the art that the within disclosuresare exemplary only and that various other alternatives, adaptations andmodifications may be made within the scope of the present invention.Accordingly, the present invention is not limited to the specificembodiments as illustrated herein, but is only limited by the followingclaims.

What is claimed is:
 1. A process for removing undesired material from achosen substrate comprising the steps of:(a) placing said substratecontaining said undesired material in a cleaning vessel; (b) contactingsaid substrate containing said undesired material with a chosen densephase gas above the critical pressure and critical temperature of saidgas, said gas being capable of producing a cleaning effect to assist inthe removal of said undesired material from said substrate; (c) exposingsaid substrate and said dense phase gas to radiation of a predeterminedwavelength to produce a photochemical reaction that removes saidundesired material from said substrate wherein said dense phase gasenhances the removal of said undesired material from said substrate. 2.A process as set forth in claim 1 wherein said radiation comprisesultraviolet radiation within the range of 184 to 300 nanometers.
 3. Aprocess as set forth in claim 1 wherein said dense phase gas is selectedfrom the group consisting of carbon dioxide, nitrous oxide, krypton,xenon, and oxygen.
 4. A process as set forth in claim 1 wherein step "b"further comprises contacting said substrate with a chosen dense phasereactive agent capable of reacting with said undesired material tothereby enhance the removal of said undesired material.
 5. A process asset forth in claim 4 wherein said chosen reactive agent is selected fromthe group consisting of an oxygen-containing compound, ahalogen-containing compound, and a hydrogen-containing compound.
 6. Aprocess as set forth in claim 5 wherein said chosen reactive agent isselected from the group consisting of oxygen, oxone, and hydrogenperoxide.
 7. A process as set forth in claim 6 wherein said chosenreactive agent comprises ozone in a supercritical state.
 8. A process asset forth in claim 6 wherein said substrate comprises spent carbonhaving contaminants adsorbed thereon and said process regenerates saidcarbon.
 9. A process as set forth in claim 4 wherein said chosenreactive agent is formed in situ in said cleaning vessel.
 10. A processas set forth in claim 4 wherein said reactive agent is mixed with acarrier fluid.
 11. A process as set forth in claim 1 wherein saidundesired material is selected from the group consisting of an organicmaterial, an inorganic material, and an ionic material.
 12. A process asset forth in claim 1 further comprising after step "c" removing saiddense phase gas containing said undesired material from said cleaningvessel and adding clean dense phase gas to said cleaning vessel.
 13. Aprocess as set forth in claim 1 wherein said substrate comprises a solidsubstrate contaminated on the surface thereof with a contaminantcomprising said undesired material.
 14. A process as set forth in claim13 wherein said substrate is further contaminated in the intersticesthereof.
 15. A process as set forth in claim 1 wherein said substratecomprises a solid substance, said undesired material comprises thesurface layer of said substrate, and said surface layer is removed fromsaid substrate.